Resin films for packaging are required to have various performances such as strength, transparency, formability and the like. In the field of food packaging, in particular, the gas-barrier properties of resin films are considered an important requisite performance needed for long term storage of contents. Among resin films, there are resins composed of a single component and still having the gas-barrier properties. However, in the case where higher gas-barrier properties are required, a resin film having a resin layer laminated on such a resin film described above to further improve the gas-barrier properties, is used. Representative examples of the resin used to improve gas-barrier properties include polyvinylidene chloride (PVDC), which is a chlorinated resin, and polyvinyl alcohol (PVA), which is a water-soluble polymer having hydroxyl group. However, PVDC has problems such as acidic gas generation during an incineration treatment and the like, and thus, substitution by non-chlorinated resins is on demand. PVA is a non-chlorinated resin, and has excellent performance related to the gas-barrier properties in a dry state. However, PVA has a problem that the gas-barrier properties deteriorate upon moisture absorption, and thus is not suitable for the packaging of foodstuffs containing large amounts of moisture.
Furthermore, in addition to the above-described gas-barrier properties, the following properties, for example, are required in general from resin films.
For example, in the case of using resin films as a material for packaging under high humidity conditions, water resistance (water-insolublity) should be necessarily taken into account, along with the gas-barrier properties. In order to enhance the water resistance of PVA to improve the humidity-dependency of the gas-barrier properties, several methods to crosslink PVA with poly(meth)acrylic acid by an esterification reaction have been suggested.
For example, Japanese Patent Laid-Open No. H7-266,441 suggests a method of thermally treating a solution containing PVA and poly(meth)acrylic acid. Japanese Patent Laid-Open No. H8-41,218 suggests a method of thermally treating a solution containing PVA, poly(meth)acrylic acid and a monovalent metal salt such as hypophosphite. Japanese Patent Laid-Open No. H10-237,180 suggests a method of coating an aqueous solution containing PVA and a partial neutralization product of polyacrylic acid or polymethacrylic acid on a film, thermally treating the coated film, and then immersing the coated film in a solution containing metal. These methods attempt to improve the gas-barrier properties and water resistance by reacting the hydroxyl group of PVA with the carboxyl group of poly(meth)acrylic acid.
Japanese Patent Laid-Open No. 2001-164,174 suggests a method of coating an aqueous solution containing, in addition to PVA and polyacrylic acid, a crosslinking material such as an isocyanate compound or the like on a film, and thermally treating the coated film.
Moreover, Japanese Patent Laid-Open No. 2003-191,400 suggests, as a different approach to the improvement of the humidity-dependency of the gas-barrier properties, a composite coating film comprising PVA, poly(meth)acrylic acid, and a metal oxide resulting from hydrolysis and dehydration condensation of a metal alkoxide. This coating film has high gas-barrier properties under high humidity conditions, and thus can be used as a packing material sterilizable by boiling or retorting.
These inventions enhanced the water resistance of resin films, and could improve the humidity-dependency of the gas-barrier properties to some extent.
However, in the above-described methods of the related art, there have been occasions requiring thermal treatment at relatively high temperatures in order to obtain sufficient gas-barrier properties and water resistance. In these cases, improvement was still needed in the aspect of productivity, and a high thermal treatment temperature often caused coloration of the resin and impairment of the resin appearance.
Furthermore, in recent years, there appeared fields requiring even better gas-barrier properties, such as, for example, electronic materials for displays and the like, in addition to the packaging materials. Technologies to cope therewith are demanded. However, when the resins of the related art were to be applied to such fields, there still was room for improvement in the aspect of substrate deformation due to high temperature, in addition to the coloration.
Also, with respect to the use as electric/electronic materials as described above, the resins may need, for example, high transparency and abrasion resistance, in addition to the water resistance and gas-barrier properties required by conventional packaging materials. Transparency is necessary for confirming the contents inside by looking through. Abrasion resistance is necessary for preventing the products from being damaged.
[Patent Document 1] Japanese Patent Laid-Open No. H7-266,441
[Patent Document 2] Japanese Patent Laid-Open No. H8-41,218
[Patent Document 3] Japanese Patent Laid-Open No. H10-237,180
[Patent Document 4] Japanese Patent Laid-Open No. 2001-164, 174
[Patent Document 5] Japanese Patent Laid-Open No. 2003-191,400